Upsert tries to insert NOT NULL columns as NULL

When upserting a record in a database column is not nullable and has a default value, ActiveRecord::Upsert attemps to insert a nil value. This raises an exception:

PG::NotNullViolation: ERROR: null value in column "uuid" violates not-null constraint

The table looks like this:

```sql
CREATE TABLE public.defaulting_records (
id bigint NOT NULL,
uuid uuid DEFAULT public.gen_random_uuid() NOT NULL,
name character varying,
created_at timestamp without time zone NOT NULL,
updated_at timestamp without time zone NOT NULL
);
```

And the SQL generated for the upsert is as follows:

```sql
INSERT INTO "defaulting_records" ("uuid", "name", "created_at", "updated_at") VALUES ($1, $2, $3, $4) ON CONFLICT ("id") DO UPDATE SET "name" = $5, "updated_at" = $6 RETURNING *, (xmax = 0) AS _upsert_created_record
```

Failing test available in this branch: https://github.com/andrewclink/active_record_upsert/tree/upsert_null

Start with the failing test in the upsert_null branch and compare its generated SQL with the SQL shown in the issue. Confirm the upsert behavior for a NOT NULL column with a database default, then update the test or implementation as needed so the default is used instead of inserting NULL.

Written by the indexing model from the issue text.